Percolation? Heat soak? Don't drive during summer??

Yes I disconnected the old ecm and installed the new one. I was thinking maybe it was getting a bad ground on the body due to the paint (even though it was in the same spot).


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You definitely want the ECU to be grounded properly.
Some of the ECU'S are painted front and back. Take a screwdriver and scrape the paint off the 2 bolt mounting areas on the back of the ECU. And then, take the same screw driver and scrape the paint off the car where the ECU mounts. Only scrape the paint off the car where the ECU will cover.
This will give you a nice ground.

On my slant car, I added a ground jumper wire, for a good ground.

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